This invention relates generally to calibration and alignment methods and system, and, more particularly to methods and system for calibration and alignment of imaging devices operating in time delay integration (TDI) mode.
CCDs (charge coupled devices) are utilized as imaging sensors. A two dimensional CCD sensor includes a two dimensional array of photodiode areas and structures (CCD registers) to transport the charge from each photodiode across the array and to xe2x80x9creadxe2x80x9d the charge at one location. The basic charge coupled structure consists of a metal-insulation-semiconductor (MIS) capacitor structure in which minority carriers are stored in a xe2x80x9cspatially defined depletion region,xe2x80x9d also called xe2x80x9cpotential wellxe2x80x9d at the surface of the semiconductor material. A CCD includes arrays of the metal-insulation-semiconductor (MIS) capacitors. The charge (charge packet) is moved along the surface by moving the potential minimum. The movement is accomplished by applying a proper sequence of clock voltage pulses.
Two dimensional (area) CCD sensors can be utilized as time delay integration (TDI) sensors for linear imaging. Time delay integration (TDI) is an integration and readout mode that allows the acquisition of a line image from a moving image. In this mode of operation, a linear image is scanned across one of the sensor directions (from top to bottom, for example). The CCD registers along the direction of motion are clocked to ensure that charge packets are transferred at the rate that the image is moving at and in the same direction as the image motion. This results in the summation of the charge from the sensing areas (pixels) along the direction of motion. Sensitivity is increased at the expense of resolution.
However, line scan imaging systems that employ Time Delay Integration (TDI) Charge Coupled Device (CCD) sensors are difficult to align and calibrate for optimal image quality. The difficulty arises due to the two-dimensional characteristic of TDI CCD arrays.
There is a need for methods and systems for calibrating and aligning Time Delay Integration (TDI) Charge Coupled Device (CCD) sensors.
The present invention provides methods and systems for calibrating and aligning Time Delay Integration (TDI) Charge Coupled Device (CCD) sensors.
Applying an embodiment of the method of this invention, a TDI sensor for linear imaging (line sensor) is calibrated by generating a two dimensional image from the line sensor, analyzing the two dimensional image, and calibrating the line sensor based on the analysis. An alignment correction can then be generated, the correction applied to the line sensor placement and the line sensor re-tested.
A calibration system of this invention includes means for generating a two dimensional image from the TDI line sensor, means for analyzing the two dimensional image, and means for calibrating the line sensor based on the analysis of the two dimensional image. Since the TDI line sensor is an area sensor, the two dimensional image results from obtaining the signal from each pixel. In one embodiment, the TDI array clocking arrangement is modified to obtain the signal from each pixel. Analysis methods for the two dimensional image include, but are not limited to, visual analysis of the image and image acquisition and processing.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims.